Non-Ultrasound Approach
Measurement of Fundal Height
As the uterus grows it increases approximately a centimeter per week after 20 weeks of pregnancy. Therefore, at 28 weeks it should measure 28 cm, at 32 weeks 32 cm, etc. While this measurement is useful, it is fraught with potential measurement problems in women who are overweight and the inability of the physician to identify the top of the uterus for proper measurement. Measurements are also influenced by the position of the fetus in the uterus, especially if is lying transversely. The rule-of-thumb the obstetrician uses is a lag of growth of greater than 2 cm for the gestational week. Therefore, if the fetus is 34 weeks and the fundal height measurement is 32 cm, a concern should be raised regarding potential IUGR.
Maternal Weight Gain
Each obstetrician should have available in your prenatal record a graph that plots the net weight gain throughout pregnancy. While this is useful, it often is a late manifestation of IUGR and is not a good screening test.
Clinical Diagnosis of IUGR
Using the above two screening methods, obstetricians identify less that 50% of fetuses with IUGR.
Ultrasound Approach
If using the above clinical approaches identifies less than 50% of fetuses with IUGR, one has to ask the question, "Is there a better method?" The answer to this question is YES, if one uses diagnostic ultrasound.
What are the Different Ultrasound Approaches Physicians May Use to Diagnose IUGR?
Approach I.
The obstetrician only uses ultrasound when IUGR is clinically suspected by inadequate fundal growth and/or maternal weight gain. The typical scenario is as follows:
The obstetrician does not obtain an ultrasound in the first or second trimesters of pregnancy. In the third trimester IUGR is suspected and an ultrasound is ordered. The results identify the fetus to 31-week size when the patient is thought to be 34 weeks of gestation. The problem with this approach is that without an ultrasound before 20 weeks of gestation the actual age of the fetus is uncertain, even if the patient remembers her last menstrual period and feels that it is accurate. The problem now is whether the fetus has IUGR or is really 31 weeks of gestation? If the latter is the situaion, then the "normal " fetus is exposed to unnecessary testing.
Approach II.
The obstetrician realizes that even with an accurate last menstrual period, many women do not conceive 14 days after their last menstrual period. Because of this the physican routinely obtains an ultrasound during the first or second trimesters of pregnancy to assign a gestational age and due date. The typical scenario is as follows:
The physician orders an ultrasound at 18 weeks of pregnancy and determines that the gestational age is 2 weeks greater than the patient's last menstrual period would suggest. He changes the due date. In the third trimester IUGR is suspected and an ultrasound is ordered. The results identify the fetus to be 31-week size when the patient is 34 weeks of gestation based upon her second trimester ultrasound examination. Because there is lag in growth of 3 weeks, IUGR is diagnosed and the patient undergoes surveillance that results in a premature delivery at 35 weeks of gestation.
Approach III.
The obstetrician believes that it is better to identify the fetus who is undergoing early growth restriction instead of waiting until severe growth restriction develops. This approach is popular in Europe in which patients receive a minimum of three ultrasound examinations during their pregnancy. The first ultrasound evaluation occurs between 10 and 14 weeks of gestation to determine the age of the fetus. The second examination occurs at 22 weeks of gestation to identify birth defects and measure blood flow to the uterus using Doppler ultrasound. If blood flow to the uterus is abnormal, the risk for IUGR is increased as well as other potential problems. If the first and second trimester examinations are normal, the fetus is reexamined between 32 and 34 weeks of gestation. If abnormal growth is identified, surveillance and treatment are initiated. In some cases when growth restriction is identified in its early stages, the fetus often reverses its growth lag when the mother is placed at complete bed rest. Of the three approaches, the use of ultrasound in the first, second, and third trimesters of pregnancy is the one preferred by myself because if offers optimal care for the identification and prevention of IUGR.
How Would I Use Ultrasound to Evaluate a Patient For IUGR
The following are considerations when screening a patient for IUGR.